What should I do to find out if a person is compatant to sign Living Wii and power of attorney?

If there is a question about whether someone is competent to sign a Power of Attorney, the best way to answer the question is to have the person evaluated by a social worker, psychiatrist, or other provider who specializes in working with elderly persons. However, your brother will have to agree to any such evaluation. If you really believe that your brother is not competent, and if you also believe that your brother's wife is doing something she shouldn't be doing and that he is in danger physically or financially as a result of her actions, then you might be able to pursue a conservatorship. If you think that's possible, contact an attorney who works with contested guardianships and conservatorships, because your brother and his wife may well not take well to your trying to get a conservatorship over him.

While elder law attorneys are ethically required to administer a screening exam before being hired or presenting documents for signing, most estate planning attorneys do not even do that.  A physician's examination can lead to an application for guardianship and then to a judge determining whether a person has legal capacity, including legal capacity to contract (sign a power of attorney) or issue a "living will."  While almost everyone has legal capacity to do the latter (It's their body), legal capacity to contract is the highest level of capacity.